1. Field of the Invention
The present invention relates to a separation apparatus having a water-oil separator and associated water leg for separating gas, water and oil mixtures where the mixture contains a significant amount of water and a small amount of oil to be separated and recovered from the water. This invention is generally intended for use in treating fluid streams relating to petroleum oil and gas production.
2. Description of the Related Art
The present invention improves over the separation apparatus taught in Applicant's U.S. Pat. No. 5,073,266. The teaching of that patent is hereby included by reference.
Specifically, the present invention includes improvements in having internal inlet piping that feeds fluids to an engineered degassing boot, in having an engineered degassing booth that is more effective in removing entrained gases from the incoming fluid stream, in having an umbrella shaped upper baffle instead of an inverted umbrella shaped upper baffle, in having an improved oil collection bucket or weir, in having an improved water leg design, and in having a water leg with a functional height that is externally adjustable to make it easier to regulate the level of the oil-water interface within the separation vessel.
It should be understood that the oil-water interface may not be a clearly defined line due to the fact that the separation zone contains a mixture of oil and water.
In applications where inlet fluids include significant quantities of gas, it is necessary to install a system designed to separate the gas from the liquids in order to prevent gas evolution related mixing in the liquids separation section downstream. The generic system is identified as a “de-gassing boot”; however these have always been and still are just empty vertical pipe liquids-gas “separators” where no engineering expertise was ever applied. Offsetting this, the present invention utilizes a similar vertical vessel with engineered internals. These internals absorb the momentum of the inlet fluids, redirecting their flow into a progressively thinner layer of liquids so the entrained gas can more readily escape.
Vertical flow diverters provided within the de-gassing boot in association with the fluid inlet spread the liquids out inside the de-gassing boot. The liquids then gravity flow down an inclined baffle provided within the de-gassing boot and at the end of the inclined baffle the liquids cascade onto an opposing inclined plate provided within the de-gassing boot which serves to further thin the stream for even more efficient gas-liquids separation.
Also, the present vessel is unique to the de-gassing boots on most previous vessels including the vessel taught in the '266 patent in that the total fluids inlet and the gas outlet are designed to be installed internal to the structure to avoid the safety hazards of having to install these piping assemblies in the field at dangerous elevations above OSHA minimums. Installing this piping in the factory during fabrication simplifies field installation and eliminates concerns for the installation of field piping at heights. This also eliminates the need for external insulation, as the inside piping prevents winter time freeze ups.
The invention of the '266 patent mechanically failed within two years of being placed in operation because the upper baffle, which was shaped like an upside-down umbrella, filled with heavy sand and solids which caused it to collapse within the vessel. The present invention has modified this design by inverting the upper baffle so that it does not fill with sand and solids.
While the vessel of the '266 patent was originally conceived as a skim tank dedicated to the skimming of small amounts of oil from large quantities of produced oilfield water, many users have recently requested that it be redesigned to provide a more significant oil layer to aid in the separation of water from the stored (skimmed) oil while still incorporating the skim tank design and oil-from-water separation efficiencies.
In order to do this the vessel had to be altered by either increasing the height of the vessel to provide additional oil storage space and maintain the skimming abilities designed into the original '266 vessel or an oil collection system had to be designed into the original '266 vessel which used only a simple side-mounted nozzle on the vessel to overflow oil.
The side mounted connection provided no measure of uniform oil collection necessary for the desired oil dehydration function.
The original '266 vessel was conceived as a 20′ high vessel. In order to accommodate crude oil dehydration, that height had to be increased 4-10 feet, depending on the ease of dehydration according to Stokes' Law.
In order to accomplish the dehydration process, the crude oil layer must be 1) uniformly distributed, 2) quiescent, and 3) remain in the vessel for the maximum period of time. All of these factors are dependent on distribution of the incoming crude oil throughout the cross section of the vessel via a newly redesigned high efficiency inlet distributor, and the newly designed oil collection system (s).
When crude oil is light and water-from-oil separation is comparatively easy, the first of two different oil collection systems is used. It is a large diameter spillover bucket type collector concentrically located in the center of the vessel 1-2′ from the top. The diameter is fixed at 5′ which produces a 15.7′ spillover weir. With only 2″ of crest height (oil level above the weir edge) this engineered oil collector accommodates instantaneous flows of up to 51,360 barrels per day. With a 4″ crest height, the flow can reach 145,440 barrels per day on an instantaneous or sustained basis into the vessel which means it is virtually impossible to flood the vessel and equally impossible to overflow crude oil out of the vessel and into the environment. The result is that less oil is wasted and more stock tank oil is sent to the refinery.
When crude oil is heavier, more care must be taken to assure its retention time in order to produce the desired dehydration results. In this case the vessel's oil collector described above is replaced with a circumferential ring trough. Adding this same feature to the vessel provides the desired results, completely dehydrating the inlet crude, making it ready for sale.
The '266 patent touted the use a water leg wherein the water flowed into and up the inside of two concentric pipes. Later hydraulic engineering studies proved the fallacy of this approach as the emphasis on maintaining a more and more constant contact elevation grew. It finally became clear that the original design needed to be reversed to minimize the effect varying flow rates have on the pressure drop through the water outlet piping and water leg.
The reason is not obvious, so it is worth an explanation. Remembering that this vessel is designed to remove small quantities of crude oil from large quantities of produced oilfield waters, it is important to understand the condition this oil is likely to be in. This oil remains in the water reaching this vessel because the oil droplets are exceedingly small. According to Stokes' Law of separation, smaller droplets separate at the square root of the separation rate of droplets twice their size. These small droplets have such a slow separation velocity that until they find an area of almost no movement, they stay entrained and dispersed in the water. The present vessel design gives these droplets that area, so the smallest of those oil droplets can accumulate. However, any movement of this area results in a high degree of re-entrainment of these small and fragile oil droplets.
Since the water outlet piping and the external water leg determine the variation in the level where these most fragile oil droplets are known to accumulate, this designer began to focus on minimizing any and all elevation or level changes in this layer. To do so the piping had to be enlarged and the water leg itself had to be both enlarged and redesigned.
The alteration moves the water out of the vessel through a much larger pipe, since fluids dynamics studies showed this outlet pipe to be a serious bottleneck restricting flow. By enlarging the outlet pipe the pressure drop is dramatically diminished, thus having the least possible influence on the oil-water contact elevation.
Then, the flow in the water leg was reversed so the water leaving the vessel flows into and up the annulus between two pipes where the friction loss is least, and therefore the pressure drop is minimized. The smaller the overall pressure drop through the water outlet piping and water leg, the less the movement at the oil-water interface. The result is far less re-entrainment of separated oils into the effluent water.
The '266 patent touted the use a water leg (e.g. a process monometer to control the oil-water contact elevation in a tank or vessel) which employed an internally removable internal part so it could be lengthened or shortened as needed to raise or lower the oil-water interface inside the body of the vessel. The removable part was a friction fit spigot shaped pipe pushed into an opposing angular bell ended pipe. Joining the spigot and the bell made for a junction which was sealed using waterproof grease. The bell and spigot connection and the grease allowed the removable part to be removed. To remove it an operator had to remove a very large and heavy flange to gain access to the insides of the water leg so the upper bell-portion could be removed. Once removed it could be shortened, or a new longer one could be made, thus shortening or raising the spillover height of the water leg itself, and correspondingly, raising or lowering the oil-water interface inside the vessel as was generically known to be necessary to optimize oil recovery and water quality improvements.
Over time it became apparent that working on the bell and spigot fittings was so arduous most operators did not bother. This meant fewer barrels of oil entered the economic stream, defeating the entire purpose for the vessel in the first place. The present invention replaces the bell and spigot fittings and all that was associated with them with an externally adjustable adjustment assembly.
The function of the external adjustment assembly is to provide a simple mechanism which allows the operator to make adjustments “on the fly” without interrupting his operation. Adjustments to the water leg are necessary to optimize oil recovery, and may be made on a day-to-day basis where an external adjuster is available.
The external adjuster is a comprised of an internal slip-sleeve which rides up and down on a smooth pipe section. The sip sleeve is O-ring sealed onto the smooth pipe section to avoid leakage which could/would defeat its function. The slip sleeve is connected to an external jack screw assembly to which a worm gear mechanism is connected. A hand wheel is mated to the worm gear so that turning the hand wheel clockwise raises the slip sleeve while turning the hand wheel counterclockwise lowers the slip sleeve. The worm gear is connected to the slip sleeve by a hollow light-weight rod which is lubricated through a packing gland to completely seal the water leg so no contaminants (water, oil or gas) escape into the atmosphere.
The water inside the water leg rises between two concentric pipes, and spills over into the inner pipe at the elevation set by the adjuster. A one inch change in the adjuster elevation translates to a four inch change in the water-oil contact point (elevation) inside the vessel.
The real benefit of this adjuster is that it provides a simple way for the operator to adjust the critical level in the vessel. By raising the level the operator send more separated oil to the sales oil tank, improving the cash flow of his company. These fine adjustments also improve the separation of oil from water, maximizing the efficiency of the vessel and minimizing the quantity of otherwise wasted crude oil. Crude oil not separated in the vessel often is disposed of with the waste water into deep disposal wells, where the oil is a plugging agent that tends to plug the well and prevents disposal over the long term. This can cost oil operators millions of dollars in re-drill expenses when a disposal well must be replaced.